The goal of our work is to define rules that relate external and middle-ear structure to function so that we can understand the functional significance of normal and pathologic variations in middle-ear anatomy. In this application we propose experiments designed to investigate how differences in the configuration of the tympanic membrane, ossicles and middle-ear air spaces affect middle-ear performance. The processes involved represent the most peripheral action of the middle ear. Two complimentary taxonomic groups, will be studied. (1) Four rodent species (chinchilla, gerbil, hamster, and rat) have been selected because of distinct differences in both their middle ears and their audiograms. The structural differences are in the size of the tympanic membrane and ossicles, the shape and rigidity of the malleus suspension, the relative volume of the middle-ear air-spaces, and the relative size of the accessory tympanic membrane. Physiological and anatomical measurements will quantitatively determine the effects of these structural differences. (2) The middle-ears of species in the cat family (Felidae) have great structural uniformity with large variations in size. The effects of the middle-ear air spaces in domestic cat are known to be sharply frequency dependent and are related to the two-cavity configuration of the spaces that is characteristic of cats. Using skulls from museum collections we will make structural measurements on (essentially) all 36 species of this family to determine quantitative rules that relate dimensions of the ear to skull size. For a subset of these (about 12 species) more detailed reconstructions of the middle ear will be made from CT scans of the skull, which will yield three dimensional descriptions from which acoustic models can be derived. Post-mortem physiological measurements of acoustic responses will be made on specimens of 6-10 species (as available from zoos and wildlife departments), which will also be reconstructed anatomically from CT scans and from histological sections. Quantitative models relating the structural features and physiologic performance will be developed for both taxonomic groups and these models will tested by the measurements. Our aim is to provide a unified theory in which both the diverse structural configurations of these rodents and the quantitatively variable structure in Felidae are included in rules that quantitatively describe the functional consequences of both kinds of variations. This systematic approach will lay a base for a quantitative description of the effects of pathological and interspecies differences in middle-ear structure on auditory function. Such a description will replace anecdotal descriptions with a comprehensive theory that can impact the clinical understanding of middle-ear pathology and reconstruction as well as ideas about evolution of the ear.